During epidermal differentiation, keratinocytes undergo a well defined series of morphological and biochemical changes in which actively proliferating basal cells differentiate stepwise through the spinous and granular cell layers to eventually form the anuclear squames characteristic of the protective stratum corneum at the skin surface (Presland et al (1992) J Biol Chem, 267(33), 23772-23781). Each epidermal layer is characterised by the expression of specific biochemical markers of which the keratin intermediate filament proteins, K5/K14 and K1/K10, predominate in the basal and spinous layers, respectively (Presland et al (1992) J Biol Chem, 267(33), 23772-23781).
Granular cells are characterised by the expression of profilaggrin. The profilaggrin gene encodes a high molecular weight phosphorylated polyprotein, composed of a number of related but nonidentical filaggrin repeats. Peptide mapping and sequencing studies have revealed that filaggrin units are separated by short linker peptides which are removed during proteolytic processing (Presland et al (1992) J Biol Chem, 267(33), 23772-23781). Like its rodent counterparts, the coding region of the human profilaggrin gene contains no introns within the repetitive portion of the coding region.
Phosphorylated profilaggrin is non-functional and accumulates as F-keratohyalin granules late in epidermal differentiation (Gan et al (1990) Biochemistry, 29, 9432-9440). During the transition from the granular to the terminally differentiated cornified cell, profilaggrin is dephosphorylated and proteolytically processed to yield filaggrin monomers. Filaggrin participates in the aggregation of keratin intermediate filaments into the dense macrofibrils characteristic of the stratum corneum (Presland et al (1992) J Biol Chem, 267 (33), 23772-23781). Profilaggrin may also play a role in maintaining epidermal hydration through the degradation of filaggrin to free amino acids (Presland et al (1992) J Biol Chem, 267(33), 23772-23781). The free amino acids form part of the Natural Moisturising Factors (NMF) of stratum corneum. NMF maintains the hydration of the skin and hence its condition.
The profilaggrin gene is located on Chromosome 1q21 as part of the cluster of genes known as the Epidermal Differentiation Complex (EDC) (Mishke et al (1996) SID 106(5): 989-992). Many of these genes encode products which are believed to contribute to stratum corneum structure and function. The profilaggrin gene has been reported to be polymorphic in size due to allelic differences in the number of filaggrin repeats located in exon 3 (Gan et al (1990) Biochemistry, 29, 9432-9440). Within the human population 3 length variants of the profilaggrin gene have been identified, encoding multimers of 10, 11 or 12 repeats. It has been shown that both profilaggrin alleles are expressed at approximately equal levels, ie, that expression from the profilaggrin gene is bi-allelic (Nirunsuksiri et al (1998) Journal of Investigative Dermatology, 110(6), 854-861)
The allelic differences of profilaggrin genes in individuals affected by ichthyosis vulgaris (IV), a scaling skin disorder inherited as a dominant trait, were compared to unaffected, related or age- and sex-matched normal controls (Nirunsuksiri et al (1998) Journal of Investigative Dermatology, 110(6), 854-861). Estimation of the size and number of repeats was performed utilising the EcoRV restrictions sites that flank the entire coding region. The number of filaggrin domains was shown to vary between 10 and 12 in both IV and control individuals and no obvious difference in the distribution of alleles was seen between the two groups (Nirunsuksiri et al (1998) Journal of Investigative Dermatology, 110(6), 854-861) suggesting that the profilaggrin genotype of an individual has no influence on the skin condition of that individual. This view is further supported by Gan et al (1990, Biochemistry, 29, 9432-9440) who note that it would appear that normal terminally differentiated human epidermis is not critically dependent on the precise amount of functional filaggrin produced from the precursor gene.
Against this background it has been surprisingly shown a correlation between the number of filaggrin repeats and the predisposition to dry skin. We have shown that there is a relationship between profilaggrin genotype and ability of skin to withstand surfactant challenge (ie, predisposition to detergent-induced erythema). It has also been demonstrated that there is a direct correlation between the number of filaggrin repeats and the production of NMF. An individual's ability to produce NMF and/or predisposition to a skin condition such as dry skin, dandruff and/or detergent-induced erythema can be determined by identifying the profilaggrin alleles present in their genome. Individuals can be grouped during clinical trials based their profilaggrin genotype. Individuals can be matched with appropriate cosmetic products based on their profilaggrin genotype.